In the past, full duplex communication over two-wire lines has been achieved by the use of modems (modulator-demodulators) which use frequency division multiplexing in binary level asynchronous frequency shift keying to obtain full duplex transmission at data rates of 300 bits per second. A typical system is disclosed in U.S. Pat. No. 3,655,915.
For many years, full duplex transmission at data rates of 1,200 bits per second has become accepted in the industry. Such systems provide synchronous data transmission using double sideband, suppressed carrier, quadrature amplitude modulation. A typical system of this type is disclosed in U.S. Pat. No. 3,937,882. Other 1,200 bit per second modems have utilized frequency division coupled with four-level phase modulation of the carriers.
More recently, a method of transmitting 2,400 bits per second has come into use which employs frequency division in a complex amplitude and phase modulation scheme of 16 levels.
All these prior art systems have been relatively wasteful of bandwidth, relatively subject to noise interference, and/or relatively subject to phase jitter or distortion which limited the maximum transmission speed over a typical 3,000 Hz voice grade telephone channel.